The proposed studies will examine the mechanisms of thrombin-induced increase in pulmonary vascular endothelial permeability. Studies will assess at cellular and organ levels the mechanistic basis by which alpha-thrombin increases in pulmonary capillary filtration coefficient (hydraulic conductivity) and the permeability to albumin. The basic hypothesis is that binding of alpha-thrombin and/or its proteolytically site activates second messenger signal transduction pathways. The resultant generation of second messengers, i.e. inositol phosphates and diacylglycerol, and the mobilization of intracellular calcium and activation of protein kinase C mediate the increases in pulmonary microvascular endothelial permeability. Studies will examine how the interactions between these two essential transduction pathways mediate the increase in permeability. Cyclic nucleotides, in particular cAMP, have been shown to modulate the increases in endothelial permeability in response to various inflammatory mediators. The studies will also address how cyclic AMP influences thrombin-mediated endothelial permeability increase. The proposed studies will use approaches in cellular physiology such as assessment of endothelial barrier function as well as lung vascular physiology such as the assessment of lung vascular permeability alterations. The combination of in vitro approaches with intact pulmonary circulation studies will provide a more complete description of the factors regulating endothelial permeability. An understanding of the cellular basis by which endothelial permeability is increased and is regulated may have important implications in the pathogenesis of lung vascular injury as induced by a variety of mediators.